1. Field of the Invention
The present invention relates to a capacitor using a solid electrolyte, a circuit board with built-in capacitor, and a method of manufacturing the capacitor and the circuit board with built-in capacitor.
2. Description of the Related Art
A capacitor using a solid electrolyte has a structure in which at least a first electrode, a dielectric layer, a solid electrolytic layer and a second electrode are provided in this order. In conventional way for manufacturing such a capacitor, the dielectric layer is formed on the surface of an anode, which turns out to be the first electrode, by using a method such as an anodic oxidation. Thereafter, the solid electrolytic layer is formed, and a carbon paste or a silver paste is applied onto the surface of the solid electrolytic layer, thereby forming a cathode to be the second electrode such as shown in Japanese Patent Publication JP-A-01-32621.
In the case in which a polypyrrole electrolytic polymerization film is used as the solid electrolytic layer, a polypyrrole layer is formed on the surface of the dielectric layer by a chemical polymerization and a conductivity is endowed to the surface of the dielectric layer, and then, an electrolytic polymerization is carried out.
However, a chemical polymerization film is deposited on the surface of a dielectric layer by a chemical reaction. For this reason, the thickness of a film is apt to be varied. Irregularity portions caused by such a unevenness in the thickness of the film remain on the surface of the electrolytic polymerization film after the formation of the electrolytic polymerization film. When a silver paste and a carbon paste are to be sequentially formed on the surface of the electrolytic polymerization film, therefore, the irregularity portions are eliminated by applying an Ag paste thickly. As a result, there is a problem in that the thickness of the capacitor is increased. Referring to the chemical polymerization film, moreover, a capacitor having a higher reliability can be obtained with an increase in the thickness of the film. Therefore, it is preferable that the chemical polymerization film should be formed thickly. For this purpose, it is necessary to repetitively carry out the chemical polymerization. As a result, there caused such a problem that the number of steps is increased, and furthermore, a unevenness in the thickness of the chemical polymerization film is further increased. In the case in which a capacitor is fabricated in a wiring board to constitute the circuit board with built-in capacitor, particularly, the thickness of the board is correspondingly increased if the thickness of the capacitor is increased, which is not preferable.